The Review of Laser Engineering

Nevshupa R., Hiratsuka K., Tukhbatullin A., Sharipov G.

This work seeks to summarize recent advances in experimental studying of triboluminescence and elucidate the basic mechanisms whereby triboluminescence is excited.

Gnecco E., Khaksar H., Mazo J.J.

In this chapter we describe two representative applications of atomic force microscopy to investigating early-stage wear on polymeric surfaces and layered materials on the nanoscale. Ripples and exfoliated flakes or chips are the most typical surface structures obtained in each case. On polymers the ripple formation can be interpreted within the Prandtl-Tomlinson mechanism for atomic-scale friction with the evolving surface profile defining the energy landscape sensed by the scanning probe. If the scratching is repeated, nanoplastic particles are extruded from the crests of the ripples and displaced from the places where they were formed. Layered materials are exfoliated all along the scanned track, with the wear products either folded or bent depending on the thickness of the worn material. On multilayer surfaces, but not on monolayers, stick–slip is also observed in the friction force signal acquired while scratching.

Tripathi M., Iyengar S.A., Hasan-ur Rahman M., Gadhamshetty V., Ajayan P.M., Dalton A.B.

The significance of graphene and other 2D materials as solid-state lubricants has been known for a decade. These atomically thin sheets contain sensitive surface atoms that are responsible for unique frictional characteristics that mainly depend on the nature of the underlying substrates. Thus, interfacial interactions between atomically thin sheets and engineered surfaces enhances interfacial adhesion force and induces strain in the sheets are useful for tuning the friction behaviour. This degree of regulation offers phenomenal advantages in nanoscale electromechanical systems and nanoscale robotics, where a moving mechanical system is needed. The present chapter discusses the importance of straining 2D materials for friction force regulation through engineered surfaces. Several sophisticated methodologies for preparing textured surfaces are highlighted, and modern characterisation techniques, including machine learning tools, which are useful for analysing strain and mechanics in 2D materials, are discussed. Using graphene as a case study, several results from nanoscale friction force microscopy on engineered surface are presented.

Hod O., Urbakh M., Berman D.

Structural superlubricity, a state of ultra-low friction and wear arising from incommensurability between contacting surfaces, is an intriguing physical phenomenon that holds promise for the significant reduction of energy loss and material damage in mechanical systems. One of the most prominent realizations of superlubric motion is demonstrated for nano- and micro-scale heterogeneous layered material contacts and their twisted homogeneous counterparts. On the route to scaling up superlubricity stand a few obstacles. In this chapter, we focus on the effect of grain boundaries, which inevitably emerge in large-scale layered material contacts, on their frictional properties. New frictional mechanisms associated with grain boundaries, such as shear induced buckling and unbuckling of corrugated dislocations and moiré superstructure scattering, are discussed. These, in turn, are characterized by unique frictional behavior, including nonmonotonic dependence on normal load, sliding velocity, and temperature that can be harnessed to restore structural superlubricity at increasing length-scales.

Drummond C., Ruths M.

The Surface Forces Apparatus (SFA) has proven to be an excellent tool for studies in nanotribology. The normal load, contact area, and sliding velocity between the surfaces can be controlled and unambiguously measured with higher accuracy than in any conventional tribometer. Furthermore, an image of the surfaces in contact can be obtained as the surfaces are slid, allowing the monitoring of the real size and shape of the contact area and the distance or film thickness profile between the surfaces when atomically smooth surfaces are used. It is relatively simple to perform a comprehensive exploration of the full parameter space to determine the important variables in the frictional behavior of the system. In this chapter, the principles of operation and some experimental details of the Surface Forces Apparatus nanotribometer are described.

Gutiérrez-Varela O., Pawlak R., Prampolini G., Meyer E., Vilhena J.G.

Friction is a phenomenon which is present in our everyday life although we tend to remember it only when it is nearly absent such as when “slipping on ice”. Its presence across disparate length scales (earthquakes, car engines down to molecular machines) reminds us of its ubiquity which endows friction of an utmost practical importance. Therefore, attempts to control it are almost as old as civilization and intrinsically tied to our technological progress. Interestingly, during the past decades we have witnessed a growing progress in miniaturization of devices down to the nanometer scale. “Special problems occur when things get small […] and it might turn out to be advantages if we knew how to design for them”, said Feynman when discussing the prospects of building “infinitesimal machinery”. To achieve this goal, and to design efficient molecular nano-engines, it becomes imperative to unveil the non-equilibrium processes governing friction and energy dissipation at a molecular level. This chapter provides a comprehensive review of recent advancements in understanding nanoscale friction and the role of internal molecular degrees of freedom in controlling energy dissipation during friction. We discuss how recent advancements in experimental techniques, particularly those linked to Scanning Probe Microscopy, have significantly enhanced our comprehension of the mechanical characteristics of individual molecules and their influence on dissipation processes. We delve into how these internal degrees of freedom facilitate control over energy dissipation, unlocking various pathways to achieve different applications at the nanoscale, such as superlubric states through molecular flexibility. Furthermore, we analyze potential applications of the energy dissipation pathways in novel mechanisms for achieving controlled locomotion of molecular machines.

Song Y., Maier S., Gnecco E., Meyer E.

This chapter reviews friction force microscopy investigations on single-asperity sliding contacts in ultra-high vacuum (UHV). The atomic-scale stick–slip observed under such conditions can be converted into a superlubric regime of motion by reducing the normal load and/or applying ultrasonic vibrations. Thermal vibrations and sliding direction (on a crystal surface) also influence the friction. The empirical Prandtl-Tomlinson (PT) model is introduced, which explains well the main experimental observations. The scenario is more complicated on two-dimensional (2D) materials, where the puckering effect explains the difference in friction observed on monolayers versus multiple layers. The moiré patterns formed on them are also influenced by elastic deformation, which can lead to significantly larger dissipation than that due to atomic stick–slip alone.

Ma M., Zheng Q.

Structural superlubricity (SSL), a state of near-zero friction and no wear between contacting solid surfaces, offers a disruptive approach to minimizing friction and wear. Recent years have seen a surge in SSL research, expanding its focus from fundamental science to practical applications, where the realization of robust microscale SSL play a key role. This chapter summarizes the recent advancements in SSL, with a particular emphasis on aspects that promote practical applications. These include SSL electrical contacts, SSL-based generators, the stability of SSL systems, and the environmental impact of SSL.

Kisiel M., Langer M., Gysin U., Rast S., Yildiz D., Meyer E., Lee D.

When two macroscopic bodies slide in contact, energy is dissipated due to friction. Sometimes it is desired, like in case of brakes in the bicycle, sometimes unwelcome—when you ask yourself why your automated coffee machine broke for the third time. In nanoscale, a tiny friction force is present when bodies in relative motion are separated by a few nanometer gap. This non-contact form of friction might be successfully measured by highly sensitive cantilever oscillating like a tiny pendulum over the surface. The elusive non-contact friction might arise due to vdW interaction, which is mediated by the long-range electromagnetic field or in many cases by fluctuations of static surface charges arising from material inhomogeneities. The huge dissipation might also originate from hysteretic switching of the studied material under the external action of the oscillating probe. In this chapter several experiments reporting on non-contact friction are discussed. First the Joule dissipation channel is discussed. Next we report on non-contact friction measurement over metal—superconductor transition, which allows to distinguish between phononic and electronic contribution to friction. Energy dissipation over a phase transition is further demonstrated on SrTiO3 crystal undergoing structural change. Next the non-contact friction due to switching of the charge density wave is discussed. Finally a energy dissipation due to single electron charging is reported on oxygen deficient SrTiO3 and topologically protected Bi2Te3 crystals. Interestingly the energy losses due to the single electron charging on Bi2Te3 surface are observed due to the protected character of the surface.

Bennewitz R.

Friction force microscopy is a key experimental method in nanotribology. The tip of an atomic force microscope is moved in contact over a surfaces and friction forces are detected as deflection of a micro-mechanical force sensor. While the method appears simple, special care must be taken to calibrate the force sensor and to understand the challenges in bridging the gap between molecular forces and macroscopic experiment. We discuss experimental procedures such as measurements of friction as function of load or of temperature, and on inhomogeneous materials. The chapter ends with an overview of dynamic measurements of friction, where the tip is oscillated laterally in contact or above the surface to probe dissipative interactions with the highest sensitivity.

Ferrario M., Righi M.C.

Friction and wear result in massive energy and environmental costs. The technologies nowadays available to reduce these costs are based on materials and intense research efforts are being devoted to improving the efficiency of lubricants. Among them 2D materials have emerged as promising alternative to liquid lubricants as they can provide extremely low friction at a potentially much lower environmental impact as they do not require the use of petroleum oils. Moreover, they are particularly suited for lubricating tribological systems where the use of liquid lubricants is not possible, such as those operating in vacuum, high-temperature or at the nanoscale. While the friction coefficients provided by the 2D materials can reach super-low values in mild conditions, higher pressures applications often suffer from the need of replenishment into the wear tracks. A smart solution to overcome this problem is represented by the possibility to synthesize the slippery layers in operando conditions through tribochemical reactions involving molecules made available in the tribological environment as gases, powders or additives in liquid media. The present chapter offers an overview on the state-of-the art knowledge on mechanochemical/tribochemical synthesis and the in-silico experiments based on ab initio molecular dynamics that can be performed to monitor in real time the formation of 2D tribofilms. Two case studies are also described that concerns the tribological synthesis of graphene and transition metal dichalcogenides layers.

Ma C., Arnold W.

In this chapter ultrasonic atomic force microscopy (AFM) techniques are discussed, which are dynamic AFMs working in contact mode that combine the excitation and detection of ultrasonic vibrations. Ultrasonic AFMs are widely used for quantitative mechanical property measurements and for non-destructive subsurface imaging. Here, we concentrate on the applications of ultrasonic AFMs in nanotribology studies. We will first introduce the working principles of ultrasonic AFMs, and then describe their applications in measuring surface properties and friction. Finally, we will summarize the use of ultrasonic AFMs to study and induce friction reduction and wear elimination.

Berkovich R., An R., Gnecco E.

Friction Force Microscopy (FFM) conducted in liquid environment proves to be a highly effective method for investigating atomic-scale friction on crystalline surfaces. It can probe friction between surface atoms while also providing sublattice resolution, opening doors to new areas of research. The observed similarity in FFM measurements conducted in liquid environments and Ultra-High Vacuum (UHV) is attributed to the lack of capillary bridges in both settings. These bridges usually increase adhesion between the scanning probe and the sample, resulting in surface wear during imaging in ambient conditions. We review various instances of nanotribological phenomena occurring on crystalline surfaces within different liquid environments—specifically water, ethanol, and ionic liquids—as studied by the authors of this chapter. We discuss the influence of the damping state of the sliding contact in the presence of liquids, which is reflected by variations in the slip length of the scanning probe. Finally, we showcase how FFM can be used to investigate sliding friction in ionic liquids. This approach allows us to probe the fascinating interplay between friction at the nanoscale and the unique nanostructures formed by confined ionic liquids.

Oo W.H., Özoğul A., Krok F., Gnecco E., Baykara M.Z.

This chapter reports on atomic force microscopy based nano-manipulation experiments performed on noble metal nanoislands (gold and platinum), which were previously shown to exhibit structurally superlubric sliding under ambient conditions on highly oriented pyrolytic graphite (HOPG). Experiments performed on oxidized platinum nanoislands on HOPG demonstrate an increase in interfacial shear stress when compared with non-oxidized islands, but not a breakdown of structural superlubricity. In addition, an effect reminiscent of contact aging is observed on a sample system that comprises gold nanoislands on HOPG, which interestingly is suppressed in the presence of environmental contamination. Nanomanipulation of gold islands is also performed on molybdenum disulfide (MoS2). Here, the high degree of commensurability at the interface does not result in superlubricity, but rather in a specific “directional locking” effect. The effect was observed not only on freshly cleaved flat surfaces but also on bilayers grown on a nanorough silicon wafer, where atomic-scale resolution of the complex cross-section of the system could be achieved using HAADF-STEM.

Gorb E.V., Gorb S.N.

As a result of evolutionary arm race between insects and plants, numerous plant surfaces that reduce insect attachment have been evolved. These surfaces provide an effective repelling effect against herbivores, sap-sucking insects and nectar robbers due to the reduction of adhesive and frictional forces in contact between the plant surface and insect attachment devices. This review summarizes literature data and own results on tribological aspects of insect-plant interactions. First, we provide a short introduction to attachment systems of insects. Second, tribological effects of three-dimensional micro- and nanoscopical epicuticular waxes of plants are demonstrated. The contact force reduction mechanisms of plant wax structures (roughness effect, contamination effect, fluid-adsorption effect, and wax-dissolving hypothesis) and their potential implications for biology, agriculture and engineering are discussed.